Cerium compound milling method using ball mill

ABSTRACT

A method of milling cerium compound by means of a ball mill using a milling medium, characterized in that ratio H b /r of radius r of a cylindrical ball mill container and depth H b  of the milling medium in the ball mill container disposed horizontally ranges from 1.2 to 1.9, and the ball mill container is rotated at a rotational speed which is 50% or less of critical rotational speed N c =299/r 1/2  of the ball mill container converted from the radius r expressed in centimeter. The milling method can be carried out in a wet or dry process, and the cerium compound is preferably cerium oxide. The method can be also applied for producing a cerium compound slurry.

TECHNICAL FIELD

The present invention relates to a method appropriate for milling ceriumoxide particles by means of a ball mill.

BACKGROUND ART

Physical factors dominating milling effect of ball mill millingequipments include dimension (radius r) and rotational speed rpm inregard to a ball mill container. In regard to beads, amount of filledbeads (this is expressed in ratio H_(b)/r of depth H_(b) of filled beadsto radius r (cm) of the ball mill container, or the ratio of the beadsto the internal volume of the container), or material, diameter andshape (spherical, cylindrical, etc.) of the beads may be mentioned.Among these physical factors, it is known that consumption power becomesmaximum and the best milling efficiency is obtained in case where theamount of filled beads which is expressed in H_(b)/r is 1.0 (correspondsto 50% based on the internal volume of the ball mill container).

However, in case where the amount of filled beads is as little as 30% orless (H_(b)/r of 0.6 or less), the balls start to slide along the innerwall of the container to cause remarkable damages to the inner wall.Therefore, in the actual production process, the amount of beads isgenerally kept to one third to half of the total volume of the ball millcontainer (H_(b)/r of 0.66 to 1.0).

In the milling by a ball mill, the balls are gradually lifted highly inthe rotational direction with the movement of the mill, and the ball isinvolved in a snowslide motion together with a plenty of balls when theballs are lifted at the position where there is no support below theballs. Consequently, the balls slide and fall on the surface of theballs and fall below the mill while they collide here and there(snowslide phenomenon).

When the rotational speed is increased, the balls come to fall like awaterfall in the space filled with vapor, rather than the snowslidephenomenon (waterfall phenomenon).

When the rotational speed is further increased, the mill comes to berotated while the balls are adhered to the inner wall of the mill due tocentrifugal force (adhesion phenomenon/adhesion state).

It is clear that no dispersion is achieved in the adhesion state (theballs do not move relatively with the mill). In addition, in the stateof the waterfall phenomenon, the balls and the inner wall of the millhave many damages, and dispersion is insufficient. Therefore, thesephenomena are undesirable states, and the dispersion of pigments iscarried out very efficiently in only the state of snowslide phenomenonwhich is regarded as an ideal state.

In regard to the rotational speed of the container, it is stated thatthe optimum rotational speed N₀=(203−0.60r)r^(1/2) wherein the unit of ris cm (RPM₀=(37−3.3r)/r^(1/2) wherein the unit of r is feet) at thepoint of which the snowslide phenomenon occurs is an ideal state in themilling by a ball mill (see, for example “Paint Flow and PigmentDispersion” written by Temple V. Patton, translation supervised by KenjiUeki., Kyoritsu Shuppan Co., Ltd., 1971, pp. 202-222). This publicationstates that the above-mentioned equation expressing the optimumrotational speed N₀ at the point of which the snowslide phenomenonoccurs is obtained in case where the critical rotational speed N_(c)=60g^(1/2)/2πr^(1/2)=299/r^(1/2), and is derived from N₀=(0.68−0.22r)N_(c)(rpm₀=(0.68−0.06r)rpm_(c) wherein the unit of r is feet). In addition,the publication states that the actual production process is generallycarried out in the amount of filled beads and the rotational speed ofthe container as mentioned above.

In addition, it is stated that the milling of aluminum hydroxide powderis carried out in a ball mill made of stainless steel having a diameterof 78 mm to 199 mm by means of steel beads having a diameter of 10.2 mm(see, for example, “Chemical Equipment” written by Sumiya Kano, HiroshiMio and Fumiyoshi Saito, 2001, No. 9, pp. 50-54). This publicationreports the test results in which the milling condition is as follows:bead-filling rate of 20 to 80% and number of revolutions of 0.6 to 1.3time the critical rotational speed. As a result of it, it is stated thatmilling rate becomes maximum when the bead-filling rate is 40 to 80% andnumber of revolutions is 80% of the critical rotational number, and themilling rate is increased with an increase in bead diameter, and themilling rate is lowered when the bead-filling rate is beyond 60%.

In the meanwhile, cerium oxide particles are widely used as polishingagent for substrates containing silica as main component, and recentlythere is a strong demand for cerium oxide polishing agent by which apolished face with a high quality can be obtained without surfacedefects such as scratch. On the other hand, it is also required stronglyto maintain a high removal rate so as not to decrease the productivity.Therefor unmilled large particles causing scratch and over-milled fineparticles causing a lowering in removal rate must be reduced in thenumber in cerium oxide particles to the utmost. That is, it is requireda production method by which the particle size distribution of ceriumoxide particles can be controlled in order to make it further sharp.

Cerium oxide particles have been finely divided by milling with ballmill using a milling medium such as partially stabilized zirconia oxidebeads or alumina beads. However, as these beads are very hard for ceriumoxide and milling condition which is generally achieved for milling itis too vigorous, particle size distribution of cerium oxide fineparticles becomes very broad.

The present invention resolves this problem and provides a millingmethod for obtaining cerium oxide particles with a narrow particle sizedistribution. The cerium oxide particles obtained according to thepresent invention have a narrow particle size distribution. Therefore,in case where it is used for polishing, it provides a polished face witha high quality without lowering in removal rate, and thus it makespossible to improve the production efficiency and lower the cost.

DISCLOSURE OF INVENTION

The present invention includes the following aspects:

-   -   as a first aspect, a method of milling cerium compound by means        of a ball mill using a milling medium, characterized in that        ratio H_(b)/r of radius r of a cylindrical ball mill container        and depth H_(b) of the milling medium in the ball mill container        disposed horizontally ranges from 1.2 to 1.9, and the ball mill        container is rotated at a rotational speed which is 50% or less        of critical rotational speed N_(c)=299/r^(1/2) of the ball mill        container converted from the radius r expressed in centimeter;    -   as a second aspect, the method of milling cerium compound as set        forth in the first aspect, wherein the milling of the cerium        compound is carried out in wet process or dry process;    -   as a third aspect, the method of milling cerium compound as set        forth in the first aspect, wherein the cerium compound is cerium        oxide;    -   as a fourth aspect, the method of milling cerium compound as set        forth in the first aspect, wherein the ball mill container is        rotated at a rotational speed which is 10% or more of N_(c);    -   as a fifth aspect, the method of milling cerium compound as set        forth in the first aspect, wherein the radius r of the ball mill        container is 5 to 50 cm;    -   as a sixth aspect, the method of milling cerium compound as set        forth in the first aspect, wherein the milling medium is        partially stabilized zirconia ball;    -   as a seventh aspect, the method of milling cerium as set forth        in the first aspect, wherein the milling medium has a diameter        of 0.3 to 25 mm;    -   as an eighth aspect, the method of milling cerium compound as        set forth in the first aspect, wherein zirconium is used in an        amount of 100 ppm to 10000 ppm based on the cerium compound in        terms of cerium (IV) oxide;    -   as a ninth aspect, the method of milling cerium compound as set        forth in the first aspect, wherein a water-soluble alkaline        silicate is added, pH of a slurry containing the cerium compound        is adjusted to 8 to 13, and then a wet milling is carried out to        obtain cerium compound covered with amorphous silica;    -   as a tenth aspect, the method of milling cerium compound as set        forth in the ninth aspect, wherein the water-soluble alkaline        silicate is lithium silicate, sodium silicate, potassium        silicate or quaternary ammonium hydroxide silicate; and    -   as an eleventh aspect, a method of producing a slurry of cerium        compound from an aqueous or organic solvent medium containing        cerium compound by means of a ball mill using a milling medium,        characterized in that ratio H_(b)/r of radius r of a cylindrical        ball mill container and depth H_(b) of the milling medium in the        ball mill container disposed horizontally ranges from 1.2 to        1.9, and the ball mill container is rotated at a rotational        speed which is 50% or less of critical rotational speed        N_(c)=299/r^(1/2) of the ball mill container using the radius r        expressed in centimeter.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention relates to a method of milling cerium compound bymeans of a ball mill using a milling medium, characterized in that ratioH_(b)/r of radius r of a cylindrical ball mill container and depth H_(b)of the milling medium in the ball mill container disposed horizontallyranges from 1.2 to 1.9, and the ball mill container is rotated at arotational speed which is 50% or less of critical rotational speedN_(c)=299/r^(1/2) of the ball mill container converted from the radius rexpressed in centimeter.

The present invention may be carried out by milling powdery ceriumcompound in dry process, or milling an aqueous or organic solvent mediumcontaining cerium compound in wet process.

That is, in the wet process, a slurry of cerium compound can be producedaccording to a method of producing a slurry of cerium compound from anaqueous or organic solvent medium containing cerium compound by means ofa ball mill using a milling medium, wherein ratio H_(b)/r of radius r ofa cylindrical ball mill container and depth H_(b) of the milling mediumin the ball mill container disposed horizontally ranges from 1.2 to 1.9,and the ball mill container is rotated at a rotational speed which is50% or less of critical rotational speed N_(c) of the ball millcontainer using the radius r expressed in centimeter.

In the present invention, cerium oxide is preferably used as ceriumcompound. Cerium oxides to be placed in a ball mill container with apolishing medium are cerium oxide particles with a particle diameter of0.1 μm or more, preferably 0.1 to 100 μm obtained by calciningcommercially available cerium carbonate in a shape of hexagonal plate ofseveral to ten-odd μm at 400 to 1200° C. In addition, commerciallyavailable cerium oxide powders with a mean particle diameter of 1 μm orless or several μm can be also used.

In the meanwhile, cerium compounds are not limited to cerium oxides, andwater-insoluble cerium compound such as cerium carbonate can be used.

As the potential of beads risen up by rotation of a ball mill containerbecomes high with an increase in the radius of the ball mill container,and the striking energy due to free fall thereof becomes high, fineparticles are apt to be obtained by over-milling. When cerium compound,for example relatively soft material such as cerium oxide is milled witha relatively hard medium such as zirconia, the range of theabove-mentioned radius r is important. The ball mill container used inthe present invention preferably has the radius r ranging from 5 to 25cm.

The amount of filled beads is set in such a way that ratio H_(b)/r ofdepth H_(b) of the filled beads to radius r of the ball mill containerranges from 1.2 to 1.9 (63 to 97% based on the inner volume), which is ahigher value than case where general milling by means of a ball mill(for example, H_(b)/r ranges from 0.63 to 1.0, 33 to 50% based on theinner volume) is carried out. This makes it possible to mill in acondition which does not occur a situation where snowslide phenomenon isrepeated, wherein the situation is regarded as an ideal condition ingeneral powder-milling.

When H_(b)/r is set within the range from 1.2 to 1.9, material to bemilled (cerium compound in a dry-milling, an aqueous or organic solventslurry containing cerium compound in wet-milling) that is placed with amilling medium in a ball mill, is placed in an amount of the millingmedium:the material to be milled of 1:0.5 to 1:1.2 in volume ratio. Whenthe milling medium and the material to be milled are placed in thisratio in a ball mill container, the combined volume of both amounts to65 to 99.5% based on the total volume. The slurry to be milled is aslurry containing cerium compound in an aqueous or organic solvent in asolid concentration of 1 to 70% by weight.

In addition, the rotational speed of the ball mill container is 50% orless of critical rotational speed, and 80% or less of the optimumrotational speed N_(o)=(203−0.60r)/r^(1/2) that occurs a snowslidephenomenon by which dispersion is efficiently achieved. Thus, thepresent invention excludes a condition occurring a situation wheresnowslide phenomenon of beads is repeated, wherein the situation isregarded as an ideal condition in general powder-milling.

In the present invention, milling is achieved within the range of 10% to50% of the critical rotational speed N_(c). The rotational speedcorrespond to a rotational speed ranging from 20% to 80% of the optimumrotational speed N_(o)=(203−0.60r)/r^(1/2) at which a snowslidephenomenon occurs. As mentioned above, the present invention providescerium compounds, particularly cerium oxide particles having a narrowparticle size distribution by selecting a condition out of the millingcondition which it is generally regarded that milling is achieved in thehighest effect. Further, the wet milling can provide cerium oxideslurry.

As mentioned above, the milling of cerium compound in the presentinvention utilizes milling media having small particle diameter and iscarried out in a low rotational speed of a ball mill, compared with theoptimum milling condition that is normally applied for particles. Thismake it possible to narrow the particle size distribution of ceriumcompound, particularly cerium oxide when it is milled.

In a process using a sand grinder or an attritor in which beads arecompulsorily rotated with an arm or disc, the milling is carried out inthe condition of ratio H_(b)/r of depth H_(b) of the filled beads toradius r of the ball mill container ranging from 1.2 to 1.9 (63 to 97%based on the inner volume). However, it is difficult to avoid partialover-milling due to a compulsory rotation of the milling media.Therefore, a large amount of fine particles are produced, and it isdifficult to obtain cerium oxide particles with a sharp particle sizedistribution.

When the radius r of the ball mill container is over 50 cm, thepotential energy of beads risen up thereby becomes high, and thestriking energy thereof becomes high due to free fall. Therefore, it isnot preferable as over-milling occurs and the particle size distributionof the resulting milled particles becomes broad. On the other hand, whenthe radius r of the container is less than 5 cm, it is not preferable asmilled amount per batch is too small and the cost becomes very high.Consequently, the radius r of the container preferably ranges from 5 cmto 50 cm, more preferably from 10 cm to 40 cm.

When the ratio H_(b)/r of depth H_(b) of filled beads to radius r of acylindrical ball mill container is over 1.9 (97% based on the innervolume), it is not economical as milling speed is markedly lowered. Theratio H_(b)/r of depth H_(b) of filled beads to radius r of acylindrical ball mill container is preferably 1.2 to 1.9 (the amount offilled beads is 63 to 97% based on the inner volume), further it is morepreferable that H_(b)/r is 1.2 to 1.7.

The material of beads is preferably partially stabilized zirconia,alumina, mulite or silica, which is harder than cerium oxide. Amongthem, partially stabilized zirconia that little beads are worn out ismost preferable.

The size of beads is preferably 0.3 to 25 mmφ. When the size of beads isless than 0.3 mmφ, its own weight of beads becomes too light, andmilling efficiency is markedly lowered. On the other hand, when the sizeof beads is more than 25 mmφ, the striking energy of beads each otherbecomes high, and over-milling occurs locally and fine particles areeasily produced.

In case where milling is achieved by using partially stabilizedzirconia, it is not possible to avoid contamination of zirconium elementin a slurry of cerium compound after milling. When the cerium compoundis cerium (IV) oxide, zirconium element is contaminated in an amount of100 ppm to 10000 ppm based on cerium (IV) oxide. But the element ispresent in the shape of zirconia fine particle, the element itself canbe utilized as polishing agent.

The method of milling cerium compound according to the presentinvention, particularly the method for producing cerium oxide particlescan be applied for wet milling or dry milling.

In the wet process, acid such as nitric acid, hydrochloric acid, aceticacid or the like can be used as a water-soluble dispersant. In themeantime, the wet milling for a long time causes a rise in pH of an acidslurry, the pH approaches 5 that is the isoelectric point of cerium (IV)oxide. Therefore, the slurry is liable to be aggregated and lowered ingrindability.

Thus, in the process of wet milling in the present invention, awater-soluble alkaline dispersant containing silica is added to covercerium (IV) oxide particles with amorphous silica, and the resultingslurry is adjusted to pH 8-13 that is higher than the isoelectric pointof cerium (IV) oxide. Thereby, cerium (IV) oxide particles are chargednegatively, and the slurry is always kept in a dispersed state, andhomogeneous wet milling can be carried out for a long time. Thewater-soluble alkaline dispersant containing silica includes awater-soluble alkaline silicate or silica sol, such as lithium silicate,sodium silicate, potassium silicate, quaternary ammonium hydroxidesilicate, and can be added in an amount of 0.001 to 1 in a weight ratioof (SiO₂)/(CeO₂).

The material of the ball mill container according to the presentinvention includes metal such as stainless steel, iron or the like,ceramics such as alumina, mulite or the like, resin such as nylon,polyethylene, polypropylene, engineering plastics or the like.Containers made of resin are preferable taking contamination ofimpurities on milling or hardness of material into account.

Cerium compounds obtained according to the present invention have theparticle diameter measured by centrifugal sedimentation method rangingfrom 50 to 600 nm, and have a low rate of large particles over 400 nm inthe whole particles compared with those of the prior milling method.Further, the cerium compounds have also a low rate of fine particlesless than 30 nm in the whole particles. Consequently, the presentinvention can provide cerium compound particles with a narrow particlesize distribution.

In case where milling is carried out in the wet process, cerium compoundslurry that contains cerium compound with the above-mentioned particlediameter and particle size distribution in concentration of 10 to 60% byweight and that has pH of 3 to 11 is obtained by milling cerium compoundin concentration of 10 to 60% by weight with an aqueous medium of pH3-11 for 1 to 72 hours. Particularly, it is useful for producing ceriumoxide slurry from an aqueous medium containing cerium oxide.

EXAMPLES

Hereinafter, the present invention is described based on examples. Theanalytical methods adopted in the examples are as follows.

(1) pH Measurement

A pH meter (manufactured by To a DKK Ltd., HM-30S) was used for pHmeasurement.

(2) Conductivity Measurement

A conductivity meter (manufactured by To a DKK Ltd., CM-30S) was usedfor conductivity measurement.

(3) Measurement of Particle Diameter by Centrifugal Sedimentation Method

A mean particle diameter of D50 was measured with a particle diametermeasurement apparatus by centrifugal sedimentation method (manufacturedby Shimadzu Corporation, CP-3), and it was regarded as a particlediameter based on centrifugal sedimentation method.

(4) Measurement of Particle Diameter by Laser Diffraction Method

A mean particle diameter of D50 was measured with a particle diametermeasurement apparatus by laser diffraction method (manufactured byMalvern Instruments Ltd., Mastersizer 2000), and it was regarded as amean particle diameter based on laser diffraction method.

(5) Particle Diameter Determined from Specific Surface Area Measured byGas Adsorption Method

A sample obtained by drying a cerium oxide aqueous slurry in aprescribed condition was subjected to a specific surface area analyzerby nitrogen adsorption (manufactured by Quantachrome Instruments,Monosorb Type MS-16) to measure the specific surface area Sw (m²/g), anda particle diameter in terms of spherical particle (particle diametercalculated through BET method) was determined.

(6) Measurement Method of Amount of Small Particles

In 50 ml centrifugal tube, 37 g of milled slurry obtained by diluting to17% by weight of solid content with pure water was placed, the tube wascentrifuged at 3000 rpm (G=1000) for 10 minutes, and then 22.5 g ofsupernatant was taken, and dried at 110° C. to obtain powder. An amountof small particles was determined by dividing the weight of theresulting powder by the weight of solid content in the slurry prior tocentrifugation. The small pailicles were those less than 30 nm accordingto an observation with transmission electron microscope.

(7) Measurement Method of BET Method-Based Particle Diameter of LargeParticles

In 100 ml glass sedimentation tube, 115 g of milled slurry obtained bydiluting to 15% by weight of solid content with pure water was placed,and after one day, 2 ml of slurry was recovered from the bottom. Afterdrying the recovered slurry in a prescribed condition, the specificsurface area was measured similarly to the procedure in (4) and theparticle diameter based on BET method was calculated, and it wasregarded as particle diameter calculated through BET method (BETmethod-based particle diameter) of large particles.

(8) Observation with Scanning Electron Microscope

An electron microscopic photograph of a sample to be observed was takenwith a scanning electron microscope (manufactured by JEOL Ltd., FE-SEMS-4100), and the resulting photograph was observed.

(9) Measurement of Powder X-Ray Diffraction

A X-ray diffraction apparatus (manufactured by JEOL Ltd., JEOLJDX-8200T) was used for measurement of powder X-ray diffraction.

(10) Measurement of Isoelectric Point of Cerium (IV) Oxide

A slurry containing cerium (IV) oxide in 1% by weight was prepared, andthe isoelectric point thereof was measured with Zetasizer HS 3000(manufactured by Malvern Instruments Ltd.)

(11) Measurement of Removal Rate of Thermal Oxidation Layer

Film thickness of thermal oxidation layer was measured with a filmthickness analyzer NanoSpec (manufactured Nanometrics Incorporated)before and after polishing, and removal rate was determined.

Example 1

150 kg of commercially available cerium oxide having bar-shapedparticles of 0.2 to 3 μm with an observation by a scanning electronmicroscope, mean particle diameter based on laser diffraction of 3.2 μmand a specific surface area based on BET method of 128 m²/g was calcinedin 1 m³ gas calcination furnace at 1100° C. for 5 hours to obtainyellow-white powder. The resulting powder was measured with X-raydiffraction apparatus and main peaks were detected at diffraction angle2θ=28.6°, 47.5° and 56.4° which were consistent with characteristicpeaks of cubic system crystalline cerium oxide described in ASTM card34-394. An observation with a scanning electron microscope revealed thatthe calcined cerium oxide powder was aggregated particles having aprimary particle diameter of 150 to 300 nm. In addition, the specificsurface thereof was 2.8 m²/g.

Partially stabilized zirconia beads of 1 mmφ were placed in an amount of59 kg in a polyethylene container having a dimension of radius 15cm×length 34 cm (in this point, H_(b)/r=1.4, amount of filled beads was71%), and further 5.9 kg of the cerium oxide powder obtained bycalcination at 1100° C., 11.8 kg of pure water and 47 g of 10% nitricacid were placed therein. Then, milling was carried out at a rotationalspeed of 30 rpm corresponding to 39% of the critical rotational speed ofthis container N_(C)=77 rpm for 18 hours. This afforded a cerium (IV)oxide aqueous slurry having solid content concentration of 33% byweight, pH 5.9 and conductivity of 318 μm/S. The powder obtained bydrying this slurry at 300° C. had specific surface area of 7.1 m²/g andBET method-based particle diameter of 117 nm. In addition, the particlediameter thereof was 100 to 300 nm with an observation by a scanningelectron microscope, and the mean particle diameter was 260 nm accordingto centrifugal sedimentation method. Further, the proportion of smallparticles less than 30 nm was 1.5% and the BET method-based particlediameter of large particles was 140 nm. The proportion (%) that theparticle diameter of the resulting particles fell within the meanparticle diameter according to laser diffraction method±30% was 66% inthe whole particles. In addition, zirconium element was contained in1300 ppm based on cerium (IV) oxide.

Example 2

Zirconia beads of 1 mmφ were placed in an amount of 135 kg in a ballmill container having polyethylene lining with a dimension of radius 15cm×length 73 cm (in this point, H_(b)/r=1.4, amount of filled beads was70%), and further 13.5 kg of the cerium oxide powder obtained bycalcination at 1100° C. in Example 1, 27.0 kg of pure water and 107 g of10% nitric acid were placed therein. Then, milling was carried out at arotational speed of 35 rpm corresponding to 45% of the criticalrotational speed of this container N_(C)=77 rpm for 16 hours. Thisafforded a cerium (IV) oxide aqueous slurry having solid contentconcentration of 33% by weight, pH 5.8 and conductivity of 350 μm/S. Thepowder obtained by drying this slurry at 300° C. had specific surfacearea of 7.3 m²/g and BET method-based particle diameter of 114 nm. Inaddition, the particle diameter thereof was 100 to 300 nm with anobservation by a scanning electron microscope, and the mean particlediameter was 280 nm according to centrifugal sedimentation method.Further, the proportion of small particles less than 30 nm was 1.3% andthe BET method-based particle diameter of large particles was 138 nm.The proportion (%) that the particle diameter of the resulting particlesfell within the mean particle diameter according to laser diffractionmethod±30% was 63% in the whole particles. In addition, zirconiumelement was contained in 1200 ppm based on cerium (IV) oxide.

Example 3

Commercially available cerium carbonate powder having purity of 99.9%(mean particle diameter based on laser diffraction method of 38 μm) wascalcined in an amount of 1600 g in an electric furnace at 350° C. for 5hours, and then the temperature of the furnace was risen to 900° C.followed by calcination at 900° C. for 15 hours to obtain 800 g ofyellow-white powder. The resulting powder was measured with X-raydiffraction apparatus and main peaks were detected at diffraction angle2θ=28.6°, 47.5° and 56.4° which were consistent with characteristicpeaks of cubic system crystalline cerium oxide described in ASTM card34-394. An observation with a scanning electron microscope revealed thatthe calcined cerium oxide powder was aggregated particles having aprimary particle diameter of 100 to 200 nm. In addition, the specificsurface thereof was 4.6 m²/g. The isoelectric point of the cerium (IV)oxide was pH=5.

To a mixed aqueous solution of 20 g of commercially available 25%tetramethylammonium hydroxide and 165 g of pure water, 21 g of 95%tetraethoxysilane was added with stirring by disper to obtaintetramethylammonium hydroxide silicate aqueous solution being analkaline silicate having pH of 12.8, conductivity of 8110 μm/S and SiO₂concentration of 2.9% by weight.

Partially stabilized zirconia beads of 1 mmφ were placed in an amount of6 kg in a polyethylene container having a dimension of radius 6.5cm×length 23 cm (in this point, H_(b)/r=1.2, amount of filled beads was60%), and further 578 g of the resulting cerium oxide powder, 372 g ofpure water and 206 g of tetramethylammonium hydroxide silicate aqueoussolution corresponding to weight ratio (SiO₂)/(CeO₂) of 0.01 were placedtherein. Then, milling was carried out at a rotational speed of 60 rpmcorresponding to 50% of the critical rotational speed of this containerN_(C)=120 rpm for 32 hours. After milling, beads-separation was carriedout with pure water to obtain a cerium (IV) oxide aqueous slurry (A-1)having solid content concentration of 20% by weight, pH 11.9 andconductivity of 1734 μm/S. The resulting cerium (IV) oxide had theisoelectric point of pH 3.8. The powder obtained by drying this slurryat 300° C. had specific surface area of 15.2 m²/g and BET method-basedparticle diameter of 55 nm. In addition, the particle diameter thereofwas 100 to 200 nm with an observation by a scanning electron microscope,and the mean particle diameter was 113 nm according to laser diffractionmethod. The proportion (%) that the particle diameter of the resultingparticles fell within the mean particle diameter according to laserdiffraction method±30% was 59% in the whole particles. The proportion ofsmall particles less than 30 nm was 7.9% and the BET method-basedparticle diameter of large particles was 70 nm. In addition, zirconiumelement was contained in 2760 ppm based on cerium (IV) oxide.

Comparative Example 1

Zirconia beads of 1 mmφ were placed in an amount of 25.1 kg in apolyethylene container having a dimension of radius 15 cm×length 34 cm(in this point, H_(b)/r=0.66, amount of filled beads was 30%), andfurther 2.5 kg of the cerium oxide powder obtained in Example 1, 5.0 kgof pure water and 20 g of 10% nitric acid were placed therein. Then,milling was carried out at a rotational speed of 30 rpm corresponding to39% of the critical rotational speed of this container N_(C)=77 rpm for12 hours. This afforded a cerium (IV) oxide aqueous slurry having solidcontent concentration of 33% by weight, pH 5.9 and conductivity of 318μm/S. The powder obtained by drying this slurry at 300° C. had specificsurface area of 7.4 m²/g and BET method-based particle diameter of 113nm. In addition, the particle diameter thereof was 30 to 300 nm with anobservation by a scanning electron microscope, and the mean particlediameter was 290 nm according to centrifugal sedimentation method.Further, the proportion of small particles less than 30 nm was 2.5% andthe BET method-based particle diameter of large particles was 163 nm.The proportion (%) that the particle diameter of the resulting particlesfell within the mean particle diameter according to laser diffractionmethod±30% was 41% in the whole particles.

Comparative Example 2

Zirconia beads of 1 mmφ were placed in an amount of 169 kg in a nyloncontainer having a dimension of radius 37 cm×length 73 cm (in thispoint, H_(b)/r=0.42, amount of filled beads was 15%), and further 16.7kg of the cerium oxide powder obtained in Example 1, 33.8 kg of purewater and 134 g of 10% nitric acid were placed therein. Then, millingwas carried out at a rotational speed of 12 rpm corresponding to 25% ofthe critical rotational speed of this container N_(C)=49 rpm for 13hours. This afforded a cerium (IV) oxide aqueous slurry having solidcontent concentration of 33% by weight, pH 5.5 and conductivity of 248μm/S. The powder obtained by drying this slurry at 300° C. had specificsurface area of 7.2 m²/g and BET method-based particle diameter of 116nm. In addition, the particle diameter thereof was 25 to 300 nm with anobservation by a scanning electron microscope, and the mean particlediameter was 290 nm according to centrifugal sedimentation method.Further, the proportion of small particles less than 30 nm was 3.0% andthe BET method-based particle diameter of large particles was 168 nm.The proportion (%) that the particle diameter of the resulting particlesfell within the mean particle diameter according to laser diffractionmethod±30% was 39% in the whole particles.

Comparative Example 3

Zirconia beads of 1 mmφ were placed in an amount of 135 kg in a nyloncontainer having a dimension of radius 15 cm×length 73 cm (in thispoint, H_(b)/r=1.4, amount of filled beads was 70%), and further 13.5 kgof the cerium oxide powder obtained in Example 1, 27.0 kg of pure waterand 107 g of 10% nitric acid were placed therein. Then, milling wascarried out at a rotational speed of 45 rpm corresponding to 58% of thecritical rotational speed of this container N_(C)=77 rpm for 12 hours.This afforded a cerium (IV) oxide aqueous slurry having solid contentconcentration of 33% by weight, pH 6.3 and conductivity of 92 μm/S. Thepowder obtained by drying this slurry at 300° C. had specific surfacearea of 7.2 m²/g and BET method-based particle diameter of 116 nm. Inaddition, the particle diameter thereof was 30 to 300 nm with anobservation by a scanning electron microscope, and the mean particlediameter was 340 nm according to centrifugal sedimentation method.Further, the proportion of small particles less than 30 nm was 2.3% andthe BET method-based particle diameter of large particles was 160 nm.The proportion (%) that the particle diameter of the resulting particlesfell within the mean particle diameter according to laser diffractionmethod±30% was 45% in the whole particles.

Comparative Example 4

Partially stabilized zirconia beads of 1 mmφ were placed in an amount of6 kg in a polyethylene container having a dimension of radius 6.5cm×length 23 cm (in this point, H_(b)/r=1.2, amount of filled beads was60%), and further 578 g of the cerium oxide powder obtained by calciningin a similar condition as that of Example 3, 372 g of pure water and 206g of tetramethylammonium hydroxide silicate aqueous solution prepared inExample 4 corresponding to weight ratio (SiO₂)/(CeO₂) of 0.01 wereplaced therein. Then, milling was carried out at a rotational speed of90 rpm corresponding to 75% of the critical rotational speed of thiscontainer N_(C)=120 rpm for 16 hours. After milling, bead separation wascarried out with pure water to obtain a cerium (IV) oxide aqueous slurry(B-1) having solid content concentration of 20% by weight, pH 11.3 andconductivity of 1725 μm/S. The powder obtained by drying this slurry at300° C. had specific surface area of 15.0 m²/g and BET method-basedparticle diameter of 56 nm. In addition, the particle diameter thereofwas 30 to 300 nm with an observation by a scanning electron microscope,and the mean particle diameter was 113 nm according to laser diffractionmethod. The proportion (%) that the particle diameter of the resultingparticles fell within the mean particle diameter according to laserdiffraction method±30% was 43% in the whole particles. The proportion ofsmall particles less than 30 nm was 8.8% and the BET method-basedparticle diameter of large particles was 74 nm. In addition, zirconiumelement was contained in 2900 ppm based on cerium (IV) oxide. TABLE 1Item (I) (II) (III) (IV) (V) (VI) (VII) Example 1 15 1.4 30 117 1.5 14066 Example 2 15 1.4 35 114 1.3 138 63 Example 3 6.5 1.2 60 55 7.9 70 59Comparative Example 1 15 0.66 30 113 2.5 163 41 Comparative Example 2 370.42 12 116 3.0 168 39 Comparative Example 3 15 1.4 45 116 2.3 160 45Comparative Example 4 6.5 1.2 90 56 8.8 74 43

In table 1, item (I) is radius (cm) of ball mill container, item (II) isH_(b)/r ratio, item (III) is rotational speed (rpm), item (IV) is BETmethod-based particle diameter (nm) of cerium oxide aqueous slurry, item(V) is proportion (%) of small particles less than 30 nm in the wholeparticles, item (VI) is BET method-based particle diameter of largeparticles, and item (VII) is proportion (%) in the whole particles thatthe particle diameter of the resulting particles fell within the meanparticle diameter±30%.

To aqueous sols (A-1, B-1) obtained in Example 3 and Comparative Example4, ammonium polyacrylate was added in a concentration of 100% by weightbased on cerium (IV) oxide, and then polishing compositions (a-1, b-1)were prepared by diluting the resulting mixture with pure water in amanner that the solid content of cerium (IV) oxide would be 1% byweight.

Polishing by means of the prepared polishing compositions was carriedout as follows:

-   Polishing machine: a machine manufactured by Techno Rise    Corporation;-   Polishing pad: a polishing pad IC-1000 made of closed formed    polyurethane resin (manufactured by Rodel Nitta Company);-   Material to be polished: thermal oxidation layer on 4-inch silicon    wafer;-   Number of revolutions: 60 rpm;-   Polishing pressure: 500 g/cm²; and-   Polishing time: 2 minutes.

The assessment of polished faces shown in Table 2 were carried out withan optical microscope, in which the case where fine defects wereobserved was indicated by symbol (Δ) and the case where no defect wasobserved was indicated by symbol (⊚). TABLE 2 Removal rate (Å/min)Polished Face a-1 800 ⊚ b-1 750 Δ

It can be pointed out that cerium oxide aqueous slurries in Examples 1to 2 and Comparative Examples 1 to 3 shown in Table 1 have BETmethod-based particle diameter ranging form 113 to 117 nm which isapproximately equal one another. However, the comparison betweenExamples 1 to 2 and Comparative Examples 1 to 2 reveals the followings.Comparative Examples 1 and 2 having a law ratio H_(b)/r of depth H_(b)of filled beads to radius r of the ball mill container (a law fillingrate of beads) contain small particles less than 30 nm in a high rate inthe whole particles, and the large particles thereof have a large BETmethod-based particle diameter and therefore they contain a large amountof large particles. Thus, it is understood that Comparative Examples 1and 2 have a broader particle size distribution than Examples 1 and 2.

In addition, Comparative Example 3 in which a rotational speed of theball mill container was adjusted to a high value has a high rate ofsmall particles less than 30 nm in the whole particles and a large BETmethod-based particle diameter of large particles. Therefore, it isunderstood that Comparative Example 3 has a broad particle sizedistribution.

Example 3 containing tetramethylammonium hydroxide silicate aqueoussolution as dispersant has a higher rate (%) in the whole particles ofparticles falling within mean particle diameter according to laserdiffraction method±30% than Comparative Example 4 in which a rotationalspeed of the ball mill container was adjusted to a high value.Therefore, it is understood that Example 3 has a narrow particle sizedistribution. In addition, it is understood that Example 3 contains hasa narrow particle size distribution also from the facts that it containssmall particles less than 30 nm in a low rate in the whole particles andhas a small BET method-based particle diameter of large particles.Further, as shown in Table 2, it is understood from the comparison inpolishing characteristics between Example 3 and Comparative Example 4that Example 3 has a higher removal rate and provides a better qualityof polished face.

Although a relationship between removal rate and smoothness of polishedface is generally in an opposite manner, particles in cerium compoundslurry obtained according to the present invention contain smallparticles less than 30 nm in a law rate of 10% or less in the wholeparticles and particles falling within mean particle diameter accordingto laser diffraction method±30% in a high rate (%) of 50% or more in thewhole particles, thereby the present invention makes it possible toprovide a high removal rate and a good smoothness.

INDUSTRIAL APPLICABILITY

The present invention relates to a method of milling cerium (IV) oxideparticles. The milling method of the present invention provides ceriumoxide particles that contain a small amount of fine particles and largeparticles and have a sharp particle size distribution. Therefore, incase where the cerium oxide particle obtained according to the presentinvention are used as polishing agent for substrates containing silicaas a main component, such as rock crystal, quartz glass for photomask,glass hard disk or oxidation layer of semiconductor devices, polishedfaces with a high accuracy and smoothness can be efficiently obtainedwith a high polishing speed and little scratch.

Further, in case where an aqueous sol containing cerium (IV) oxideparticle covered with amorphous silica is used for particularlypolishing substrates containing silica as a main component, such as rockcrystal, quartz glass for photomask, glass hard disk or oxidation layerof semiconductor devices, it is hard to produce residues and a goodpolished surface can be obtained.

1. A method of milling cerium compound by means of a ball mill using amilling medium, characterized in that ratio H_(b)/r of radius r of acylindrical ball mill container and depth H_(b) of the milling medium inthe ball mill container disposed horizontally ranges from 1.2 to 1.9,and the ball mill container is rotated at a rotational speed which is50% or less of critical rotational speed N_(c)=299/r^(1/2) of the ballmill container converted from the radius r expressed in centimeter. 2.The method of milling cerium compound according to claim 1, wherein themilling of the cerium compound is carried out in wet process or dryprocess.
 3. The method of milling cerium compound according to claim 1,wherein the cerium compound is cerium oxide.
 4. The method of millingcerium compound according to claim 1, wherein the ball mill container isrotated at a rotational speed which is 10% or more of N_(c).
 5. Themethod of milling cerium compound according to claim 1, wherein theradius r of the ball mill container is 5 to 50 cm.
 6. The method ofmilling cerium compound according to claim 1, wherein the milling mediumis partially stabilized zirconia ball.
 7. The method of milling ceriumcompound according to claim 1, wherein the milling medium has a diameterof 0.3 to 25 mm.
 8. The method of milling cerium compound according toclaim 1, wherein zirconium is used in an amount of 100 ppm to 10000 ppmbased on the cerium compound in terms of cerium (IV) oxide.
 9. Themethod of milling cerium compound according to claim 1, wherein awater-soluble alkaline silicate is added, pH of a slurry containing thecerium compound is adjusted to 8 to 13, and then a wet milling iscarried out to obtain cerium compound covered with amorphous silica. 10.The method of milling cerium compound according to claim 9, wherein thewater-soluble alkaline silicate is lithium silicate, sodium silicate,potassium silicate or quaternary ammonium hydroxide silicate.
 11. Amethod of producing a slurry of cerium compound from an aqueous ororganic solvent medium containing cerium compound by means of a ballmill using a milling medium, characterized in that ratio H_(b)/r ofradius r of a cylindrical ball mill container and depth H_(b) of themilling medium in the ball mill container disposed horizontally rangesfrom 1.2 to 1.9, and the ball mill container is rotated at a rotationalspeed which is 50% or less of critical rotational speedN_(c)=299/r^(1/2) of the ball mill container using the radius rexpressed in centimeter.